MFC 340001:

Approved by: bcr (mentor)
Differential Revision: https://reviews.freebsd.org/D24055

CMSG_DATA(3): Use consistent variable names

The description of CMSG_FIRSTHDR used two variables (mhdr and msg) to
reference the same thing. Use msghdr consistency across the manual page
instead.

Reviewed by: bcr
Approved by: krion (mentor, implicit), mat (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D17804